This invention relates generally to switching circuitry and more particularly to switching circuitry that inherently produces undesirable energy spikes.
As is known in the art, switching circuitry is used in a wide variety of applications. One such application is in a DC-DC converter power supply. Here the level of a DC voltage is converted to a different DC voltage level by a switching regulator. More specifically, a DC voltage source is periodically coupled to the upper and lower, inductively coupled primary windings of a center tapped output transformer. In particular, during a portion of the first half of each cycle the DC voltage source is electrically de-coupled from the lower winding and is electrically coupled to the upper winding and during a portion of the second half of each cycle the DC voltage source is electrically de-coupled from the upper winding and is electrically coupled to the lower winding. The secondary of the transformer is coupled to a load, typically a resistor and parallel connected filtering capacitor and inductor, via a diode rectifying bridge network. The time duration which the DC voltage source is coupled to the primary winding in each cycle and the time duration during which the DC voltage source is coupled to the lower winding (i.e. the duty cycles of the voltages coupled to the upper and lower windings) is related to the difference between the average level of the DC voltage produced across the load resistor and the desired DC output voltage. In the steady state then, the average DC output voltage will be equal to the desired DC output voltage. One such DC-DC converter is shown in FIG. 1. A DC voltage source, V.sub.s, here a battery 10 and a capacitor C.sub.1 connected in shunt therewith, produces an output voltage E on line 11. The positive (+) potential of this DC produced voltage is coupled, via line 11, to the input of a switching regulator 12. More particularly, the produced voltage is alternatively coupled between the center and lower end A of lower winding L.sub.1 and between the center tap (CT) and upper end B of upper winding L.sub.2. That is, when switch S.sub.1 is closed by controller 14 and switch S.sub.2 is opened by such controller 14, the produced voltage E is coupled across lower winding L.sub.1. Because of the inductive coupling between an upper primary winding L.sub.2 and the lower primary winding L.sub. 1 with the winding polarity indicated by the dots (.cndot.), a voltage 2E is induced across the entire primary winding, with the voltage 2E being more positive at the, upper end B of upper winding L.sub.2 than the lower end A of lower winding L.sub.1. Likewise, during the time the voltage source V.sub.s is coupled to the upper winding L.sub.2 switch S.sub.2 is closed and switch S.sub.1 is opened by the controller 14. The effect then is to induce a voltage 2E across the entire primary winding; the polarity of such voltage, however, now being reversed; that is, the voltage at the upper end B of upper winding L.sub.2 is now more negative than the voltage at the lower end A of the lower winding L.sub.1. It should be noted, however, that the voltage source V.sub.s is coupled to either the lower winding L.sub.1 or the upper winding L.sub.2 with a duty cycle less than 50%. Thus, during a portion of each cycle, the voltage source V.sub.s will be electrically de-coupled from both the lower and upper primary windings L.sub.1, L.sub.2. During these portions of each cycle, energy stored in stray inductances, such as leakage inductance of the primary and stray inductances of the circuit wiring, will induce voltage spikes at ends A and B. If not limited in magnitude, such voltage spike can destroy the switches S.sub.1, S.sub.2, which are transistors (either bipolar or field effect here, bipolar transistors). Here, however, a voltage spike limiter 15 is provided. Such voltage spike limiter 15 is coupled to the upper and lower ends B, A of the primary winding L.sub.1, L.sub.2 of transformer T. The voltage spike limiter 15 includes a capacitor C.sub.2 and Zener diode D.sub.z, coupled in parallel with each other, and in series with the upper and lower ends, B, A, of the upper and lower windings L.sub.2, L.sub.1 respectively, through diodes D.sub.2, D.sub.1, respectively, as shown. Considering, for example, the response of the circuit when switch S.sub.2 opens. The spike voltage at the upper end B of the upper winding L.sub.2 will forward bias diode D.sub.2 and such spike voltage will rapidly charge capacitor C.sub.2 which provides a low impedance to the transient spike voltage. The voltage on the upper end B of the upper winding L.sub.2 will be clamped to an acceptable voltage level by the Zener diode D.sub.z. An acceptable voltage level would be a voltage level which would not destroy switch S.sub.1. Alternatively, when switch S.sub.1 is opened, the voltage spike at a lower end A forward biases diode D.sub.1, and the voltage spike passing through voltage diode D.sub.1 will charge capacitor C.sub.2 and will likewise be clamped in voltage level by the Zener diode D.sub.z. The clamped voltage must be greater than 2E but less than the transistor breakdown voltage of switches S.sub.1, S.sub.2. Normally, the power contained in the energy spike can be dissipated in the Zener diode D.sub.z or some other devices serving a similar function.
Completing the circuit, the secondary L.sub.S of the transformer. T is coupled to a load resistor R.sub.L through inductor L.sub.F and parallel connected filtering capacitor C.sub.F via a diode bridge rectifying circuit 16, as shown. With the polarity of the secondary and primary indicated by the dots (.cndot.), the diode bridge rectifying circuit 16 thus produces a current I.sub.L having the same directions of flow, as indicated by the arrow 19, during both the first and second halves of each cycle. A voltage divider network 18 is provided, as shown, to produce a control voltage in response to a portion of the circuit I.sub.L for the controller 14 in a convectional manner as described above so that, in the steady state, the output voltage E.sub.o will be equal to the desired output voltage.
Spike power dissipation in a medium power DC-DC converter (100-150W) at a 20 kilohertz switching frequency can be calculated by assuming one microhenry of equivalent leakage and stray inductance and one ampere of primary current with E=150 V. Spikes occur at twice the switching frequency, F. Therefore, power loss from this spikes will be 20 milliwatts (i.e. 2(1/2)LI.sup.2 (20,000) where L is the leakage and stray inductance and I is the primary current). However, consider a 4 kilowatt power supply switching at 100 kilohertz with a primary current of 40 amperes. Here the power loss will be 2(1/2)(1.times.10.sup.-6 .times.40.sup.2) 100K=160 watts. (Note that power is proportional to I.sup.2 and frequency, F). It follows then that as the power of the converter increases, and/or the switching frequency increases, a significant amount of lost power from the spike energy results.